Plastic foam articles characterized by features such as lightweightness, shock absorbing property, heat insulating property and moldability have hitherto been mainly used as packaging containers and shook absorbing materials. The plastics themselves that have hitherto been used for these products are derived from petroleum resources, and these products cause problems such that these products remain in the natural environment when buried for disposal at the time of being discarded, and these products emit harmful gases and deteriorate incinerators when incinerated for disposal, so as for such problems to emerge as social issues.
In these years, for the purpose of solving these problems, there have been demanded foam articles made of biodegradable resins such as polylactic acid, polybutylene succinate, polycaprolactone, polyethylene succinate and polybutylene terephthalate/adipate which can be degraded by moisture, microbes or the like, and can also be treated for composting in composters. Among these polymers, in particular, polylactic acid has come into use in various applications as an ideal polymer that is a plant-derived raw material produced by polymerizing lactic acid obtained by fermenting materials such as various starches and sugars, and eventually again converted into carbon dioxide gas and water so as to be environmentally recycled on a global scale.
However, these biodegradable resins are high in gas permeability, in particular, high in water vapor permeability, and hence when products susceptible to moisture, products containing moisture in large amounts or other like products are packaged, absorption and release of moisture come to be problems. Consequently, these biodegradable resins have not been practically used yet as foam articles and molded containers.
For the purpose of solving these problems, in JP-A-5-139435, for example, disclosed is a foam container made of a thermoplastic polymer composition mainly composed of polylactic acid or a copolymer of a hydroxycarboxylic acid as a container formed of a biodegradable resin.
However, compositions derived from polylactic acid and aliphatic polyesters are generally high in gas permeability except for a small number of such compositions, do not have functions such as function to prevent the oxidation of the substances in containers caused by permeation of the oxygen in the air and function to prevent the contents in containers from moisture due to water vapor permeation, and hence have never been used for preservation containers.
JP-A-6-287347 and JP-A-9-263651 describe biodegradable laminates in each of which a biodegradable non-foam layer is laminated on the surface layer of a biodegradable foam article. JP-A-2000-109045 describes a heat insulating paper cup in which a biodegradable foam article, a sheet of paper and a sheet of biodegradable film are laminated in this order.
However, the biodegradable resins used in the above-described documents are all high in gas permeability, in particular, in water vapor permeability, and hence when used as packaging containers for products incompatible with moisture, products containing moisture in large amounts or other like products, absorption and release of moisture come to be problems. Consequently, these products in the above-described documents are hardly usable for practical purposes.
Further, for the purpose of solving these problems, JP-A-2003-55569, for example, discloses a biodegradable resin composition including a biodegradable resin, a plate-like filler and a resin-like or wax-like dispersant, and a packaging container using this resin composition.
This resin composition and this packaging container each contain 2 to 70 parts by weight of the plate-like filler and 0.05 to 20 parts by weight of the resin-like or wax-like dispersant are contained in relation to 100 parts by weight of the biodegradable resin, and have a drawback that this resin composition and this packaging container become heavy because the specific gravity of the resin becomes large due to the mixing of a large amount of the plate-like filler. Additionally, this resin composition and this packaging container are poor in heat resistance, and hence this packaging container involves a problem that neither warm water nor boiling water can be substantially poured into this container and this container cannot be substantially used for heating in a microwave oven.
The present inventors have previously reported that foam containers using such aliphatic polyesters such as polylactic acid are improved in heat resistance by promoting the crystallization of the aliphatic polyesters (JP-A-2004-217288). In this document, disclosed is an improvement of the heat resistance of the containers achieved by increasing the degree of crystallinity in portions of the containers. However, the degree of crystallinity of the whole containers cannot be said to be sufficiently achieved, and the gas barrier property of the containers needs to be improved although the containers can be imparted with heat resistance.